The present invention relates generally to implantable medical devices, and more specifically, to rate-responsive cardiac pacemakers which are capable of automatically adjusting their programmable parameters according to physiologic demand.
There has been considerable amount of work done in the area of physiologically controlled rate-responsive pacemakers. Ideally, the physiological pacemaker should vary cardiac rate and, consequently, cardiac output in response to the body's physiological needs. A variety of sensors that indicate the body's state of exercise and/or stress have been proposed for controlling cardiac rate.
Probably the best known physiologic sensor has been the sinus node itself which is responsive to both sympathetic and parasympathetic stimulation (that is, stimulation which both increases and decreases the overall activity of the heart). In a patient with heart block and a functional sinus node, dual-chambered demand pacemakers were able to mimic a normal heart by tracking the sinus node and pacing the ventricles a short atrio-ventricular (A-V) delay later. By preserving A-V synchrony, cardiac output at rest was expected to increase anywhere between 10% and 30%. However, in patients whose sinus node is incompetent (that is, either too slow, non-responsive to exercise stress, or prone to atrial flutter or fibrillation), dual chamber pacemakers could not achieve the higher rates necessary during exercise. In order to maintain cardiac output at all levels of work load, an alternate physiological sensor is required to automatically adjust the stimulation rate independent of atrial activity.
Subsequently, single-chambered, sensor-driven pacemaker systems were developed to sense various physiological parameters as the basis for varying the stimulation rate. These physiological parameters include motion, temperature, Q-T intervals, respiration rate, blood pH, stroke volume, minute ventilation, mixed venous oxygen saturation, the rate of change of ventricular pressure (dP/dt), etc.
These single-chambered rate responsive pacemakers were designed to replace dual chambered devices since it was thought that the 200% to 300% increase in cardiac output associated with an increase in heart rate overshadowed the effects of the 10-30% increase in cardiac output due to maintenance of A-V synchrony. However, the lack of correctly synchronized atrial contractions will mean that the ventricular filling pressure must rise even higher than normal to maintain cardiac output. Thus, pre-existing left and/or right heart failure may be exacerbated by the loss of A-V synchrony. Furthermore, should atrial systole (contraction) occur against a closed A-V valve as a result of retrograde conduction, clinical studies have indicated that, in addition to producing an inappropriately high atrial pressure, a further reduction in cardiac output on the order of 20% may be expected when compared against random atrial contractions.
However, the selection of an appropriate A-V delay at elevated rates is not a simple task. The A-V delay is not constant at all heart rates. Nor is it constant from patient to patient or within different age groups, e.g., an adult heart is very different than a child's heart. Typically, at rest, the A-V delay may vary anywhere between 150 and 250 msec. Once the patient begins to exercise, the A-V delay is estimated to vary anywhere between 125 and 170 msec.
What is needed, therefore, is a hemodynamically responsive pacemaker which can automatically determine both the optimum rate and A-V interval to achieve maximum cardiac output independent of age, patient to patient differences, and heart rate.